1. Department of Economics, New York University, 19 West 4th Street, New York, NY 10012, NBER, and E2e (e-mail: )
2. Sloan School of Management, Massachusetts Institute of Technology, 100 Main Street, E62-520, Cambridge, MA 02142, NBER, and E2e (e-mail: )
3. Harvard University, Littauer Center M35, Cambridge, MA 02139, and Berkeley (e-mail: )